Ultrafast optical excitation of coherent two-dimensional plasmons

THz emission from two-dimensional plasmons excited by femtosecond interband optical pulses has been studied in the time domain. Different modulation-doped semiconductor heterostructures have been investigated in order to determine the driving mechanism. The excitation energy and power dependence of the amplitude of the field radiated by the plasmons is similar in all structures to that of the underlying bulk material. A sample dependent phase shift of the plasmon signal by approximately pi is observed when the excitation energy is tuned from the bulk GaAs band edge to about 100 meV above in some structures while no phase shift is observed in others. Our observations are inconsistent with recently proposed ultrafast thermalization or impulsive Raman scattering driving mechanisms, but can be at least partly explained if the plasmons are driven by an electrostatic coupling of the transverse plasmon field to transient transport and displacement photocurrents in the underlying bulk material.